When you play classical guitar, how you sound has a great deal to do with your nails, assuming you use fingernails. Here are brief suggestions aimed at improving your tone through nail care.
1. Follow the shape of your fingertip in establishing nail shape. Determine how the string meets the nail and mimic that angle with the nail file. Keep your nails short, but long enough to do the job.
2. Avoid leaving corners, which are weak, looking both from above and from the side. Keep the leading edge as thick as you can by rounding off the beveled edge made by the file on the first pass.
3. Use the multiple surfaced buffers available at drug and grocery stores to seal the leading edge. It should reflect light when well polished.
4. If, as I do, you depend on your nails for your living, and you occasionally break a nail, consider the "industrial strength" advantages of acrylic coating applied every two or three weeks. I've used acrylics for years and can recommend them for tone and volume, plus being worry-free about breaking nails. Of all the manicurists I've patronized, none is better for guitarists than Lisa of Lisa Hair and Nails, 404 S. 2nd at San Salvador in San José, 408-287-6015.